A polarization rotator is a device that rotates the polarization vector (E-vector) of an electromagnetic wave through a selected angle. The rotation of the E-vector, usually by 90.degree., is a frequent requirement in numerous applications, especially those using a laser.
In the visible region of the spectrum, retardation plates are often used to produce the desired rotation. However, in the infrared portion of the spectrum, retardation plates are of limited value since the known birefringent materials having a high threshold for laser damage are not easily obtainable. Moreover, retardation plates produce the desired rotation only at a single wavelength.
To overcome the problems inherent in retardation plates in the infrared range, the seven and five reflection polarization rotators of the prior art were developed. The seven reflection polarization rotator was introduced by F. Keilmann in an article in Optics Communications, Vol. 14, p. 236 (1974) entitled How To Flip The Polarization Of Infrared Laser Beams, and the five reflection polarization flipper was introduced by A. R. Chraplyvy in an article in Applied Optics, Vol. 15, p. 2022 (1976) entitled Polarization Flipper For Infrared Laser Beams.
Each of the polarization rotators of the prior art requires that the E-vector of the electromagnetic wave remain either perpendicular or parallel to the plane defined by the incident and reflected electromagnetic waves to and from that surface. The seven reflection polarization rotator of the prior art also limits every reflection angle to 45.degree..
These limitations on the prior art polarization rotators have been selected to prevent the introduction of ellipticity at any point in the electromagnetic wave. Thus, the prior art devices must be properly oriented relative to the incident wave E-vector in order to have a 90.degree. rotation of the output polarization. If the incident orientation includes a deviation of x, there will be a 2x error in the rotation angle, in addition to the introduction of ellipticity in the output polarization.
It would be advantageous to have a polarization rotator that uses fewer than five reflections to increase the throughput and to simplify the construction of the device. It would be a greater advantage if the E-vector of the incident electromagnetic wave were not constrained to being either perpendicular or parallel to the plane defined by the incident and reflected electromagnetic waves to and from the first reflective surface. By removing the perpendicular or parallel orientation restraint, the device would permit the rotation of the E-vector through the desired angle regardless of its orientation to the incident wave. The present invention represents such a polarization rotator.